Dynamics of the Early and Middle Winter Atmospheric Responses to the Northwest Atlantic SST Anomalies

Abstract

The differences between early and middle winter atmospheric responses to the sea surface temperature anomalies (SSTA) in the northwest Atlantic are examined using a linear baroclinic model. Using a global spectral model, Peng et al. found a positive height anomaly in the perpetual November and a negative height anomaly in the perpetual January experiments in response to a warm SSTA over the northwest Atlantic. These height anomalies are found to be associated with the reduced Atlantic jet stream in November and enhanced jet in January. Linear model diagnostics suggest that the difference in jet stream response may induce anomalous storm track eddy vorticity fluxes, which in turn maintain the different atmospheric responses under the early and middle winter conditions. The different jet stream responses in November and January are further traced to the initial atmospheric response to a local heat source accompanying the warm SSTA. Under both the January and November conditions, the atmospheric response is dominated by an anticyclone downstream from the heat source at the jet stream level. The anticyclone is shifted northward in November, however, from its position in January. Combined with a northeast?southwest tilted jet stream in January and an east?west oriented, southward shifted November jet stream in the Atlantic, the above difference in the atmospheric responses to the initial heat source may lead to a reduced jet in November and an enhanced jet in January. The feedback between the anomalous storm track eddy vorticity fluxes and the anomaly flow induced by the heat source may further enhance the different equilibrium responses in the global spectral model.